Micellization in model surfactant systems

Formation of micelles in model lattice surfactant systems was studied by a novel methodology based on grand-canonical Monte Carlo simulations. The methodology involves combining free-energy information from a series of simulations in small systems by histogram reweighting. The solution osmotic pressure as a function of overall volume fraction of surfactant shows a sharp break at the critical micelle concentration (cmc) at sufficiently low temperatures. Studies in larger systems at appropriate values of the surfactant chemical potential are used to investigate the size distribution of micellar aggregates. The methodology allows for a clear distinction between micellization and macroscopic phase separation. Two symmetric diblock surfactants have been considered in the present work. The cmc was found to increase with increasing temperature. The enthalpy change on micellization was determined to be proportional to the chain length of the surfactant. The mean micelle aggregation numbers were found to decrease at higher temperatures and increase with overall surfactant volume fraction for temperatures near the upper limit for micellar aggregation. These observations indicate that simple geometric packing concepts for micelle formation do not adequately describe temperature and composition effects in nonionic surfactant solutions.